A new organic sulfonic acid moiety is grafted on the surface of montmorillonite (MMT) by a condensation reaction of the surface hydroxyl group of MMT with the thiol group of 3-mercaptopropyltrimethoxy silane (3-MPTMS), and simultaneously attaching sulfonic acid group by a ring opening reaction of 1,3-propane sultone. The new method produces MMT functionalized with sulfonic acid of a longer chain length in a simple one-step reaction and shows higher ion-exchange capacity and thermal stability than MMT grafted by a two-step method of 3-MPTMS condensation followed by oxidation with H2O2. Using these functionalized MMTs as inorganic fillers, organic-inorganic composite, proton-conducting membranes are cast together with Nafion (R). The composite membrane with MMT grafted with the long-chain sulfonic acid shows increased water uptake, while maintaining comparable ionic conductivity relative to a pristine Nation membrane. This leads to much enhanced performance of a proton-exchange membrane fuel cell without external humidification of reactant gases. For instance, the current density at 0.5 V of the membrane-electrode assembly (MEA) fabricated with a composite membrane containing MMT with the long-chain organic sulfonic acid is about twice that of a MEA fabricated with a composite membrane containing MMT with short-chain organic sulfonic acids and about three times higher than that with pristine Nation 212. (C) 2010 Elsevier B.V. All rights reserved.